This invention relates to the fermentation of fermentable sugars and, more particularly, to the fermentation of mono- and disaccharides produced by acid hydrolysis of carbohydrate material, preferably with recycle of non-fermented components from the fermentation back through the acid hydrolysis.
There exists in the world today an enormous demand for liquid fuels and this is being supplied almost entirely by distilled petroleum oils. It is, of course, well known that petroleum is a non-renewable resource and that finite supplies of this fuel source exist. As a result, there is now a very active search for alternative liquid fuels or fuel extenders.
Liquid fuels based on renewable resources which have considerable potential are alcohols. For instance, ethanol can be produced from almost any material which either exists in the form of, or can be converted into, a fermentable sugar. There are many natural sugars available for fermentation, but carbohydrates such as starch and cellulose can be converted into fermentable sugars which then ferment into ethanol.
Starch is, of course, one of the world's most abundant renewable raw materials and one answer would be to convert this very abundant material at low cost into fermentable sugars as feedstock for fermentation to ethanol. The fermentable sugars obtained from starch are glucose and maltose and these are typically obtained from the starch by hydrolysis, e.g. acid hydrolysis or enzyme hydrolysis. Most hydrolysis techniques which have been available have tended to be very expensive in terms of producing a feedstock for large scale alcohol production. There have been very significant improvements in the recent years in the hydrolysis of starch, particularly the continuous process described in Hughes, U.S. Pat. No. ,4,155,884. Such processes are highly economical and are well adapted for producing fermentable substrates of up to 92% fermentables or higher. Of course, in terms of maximizing ethanol production from a starch raw material source, it is desirable to have the fermentables as high as possible in the fermentation substrate.
With the typical known systems for producing ethanol from starch, e.g. using a dual enzyme system for liquifying and saccharifying the starch to glucose followed by batch fermentation, total processing times of 60 to 80 hours are usual. Fermentation times of 50 to 70 hours are commonplace. Such long total residence times result in enormous tankage requirements within the processing system when large scale ethanol production is contemplated.
In Boinot et al, U.S. Pat. No. 2,529,131 there is described a process for converting unfermentable materials into fermentable materials by acid hydrolysis. However, for their procedure to work it was necessary that the acid hydrolysis be carried out at such a high dilution that any tendency for the production of reversion products is avoided. This was accomplished by having a concentration of sugars in the unfermentables at no more than about 3% of the weight of the liquid to be treated.
U.S. Pat. No. 4,421,856, issued Dec. 20, 1983, describes a process in which stillage containing non-fermentable carbohydrate is recycled, but in this system the recycle is mixed with a hydrolyzate from a first stage hydrolysis and the mixture of non-fermentable carbohydrate and fermentable hydrolyzate is processed in a second stage hydrolysis. The content of non-fermentable sugar in the feed to the second stage hydrolysis was approximately 11% by weight.